Natural killers (NKs) are a type of innate immune cell that can directly kill tumour cells without pre-activation. It is well-known that NK cell-mediated immune monitoring not only kills tumour cells directly, but also indirectly releases cytokines to regulate other leucocytes such as macrophages, T cells and dendritic cells (DCs). 1 NK cells can destroy infected and tumour cells by releasing cytolytic granules and interferon gamma (IFN-γ). Among these molecular mechanisms, the release of granzyme B and perforin is the most effective way for NK cells to kill tumour cells. 2 Another important function of NK cells is the release of pro-inflammatory cytokines such as tumour necrosis factor-alpha (TNF-α). 3 NK cells are derived from bone marrow and are tightly regulated by IL-15